udp: Report authbind exit status as errno value if it seems appropriate

udp: Report authbind exit status as errno value if it seems appropriate

authbind-helper is documented as indicating an errno value in its exit
status.  When the exit status is small enough that it's unlikely to
have come from the runtime system, print it as an errno value instead
of using lg_exitstatus.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>